The present invention relates to a method for a thermal treatment of a non-deposition hardening type nickel based alloy which will be subjected to a high-temperature and high-pressure water or vapor and which has a remarkably improved crystal boundary etching resistance, mechanical properties and pitting corrosion resistance, in addition to the maintenance of a stress corrosion cracking resistance, and further has a bettered stress corrosion resistance in an aqueous NaOH solution.
Heretofore, as materials, for a container for giving off vapor in a nuclear reactor, which will be exposed to the high-temperature and high-pressure water or vapor, for example, at 200.degree. to 400.degree. C. and at 50 to 200 atm, and as materials used under a cooling system environment in a nuclear reactor, there are nickel based alloys such as INCOROI 800 (trade name), and INCONEL 600 (trade name) and INCONEL 690 (trade name) set forth in Table 1 below. In recent years, these alloys have further been treated by heating them at a rather lower temperature than a level (hereinafter referred to as T.degree. C.) at which a carbide is thoroughly solubilized, alternatively by further additionally specifically heating and retaining them at a temperature of 650.degree. to 750.degree. C., in order to improve the crystal boundary etching resistance and stress corrosion cracking resistance.
However, the nickel based alloys which have undergone such a conventional thermal treatment are still poor in the pitting corrosion resistance and stress corrosion cracking resistance.